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固氮作用对黑潮上游区域生态系统影响的模拟研究

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  • South China Sea Institute of Oceanophy

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A NPZD model with incorporation of a diazotroph function group was used to simulate the temporal change of ecosystem in the upstream Kuroshio Current. The modeled nutrients, chlorophyll-a, and productivity showed good agreements with the observations. The impacts of N2-fixation on nutrient concentrations, plankton biomass, and detritus were assessed by comparisons of the modeled simulations with and without diazotrophs. Our results suggested that the diazotrophs was most abundant in the summer and the fall when other phytoplankton were nitrogen limited. The mean growth of phytoplankton could increased by 64% with the input of new nitrogen from N2-fixation, leading to 30% increases of primary production, regenerated production and new production, respectively. In the summer, N2-fixation supported 50%~80% of the new production in the upper 50m, but only 10%~50% in the depths of 50~200m. This finding suggested that N2-fixation was an important source of new nitrogen for phytoplankton production above 50m in the upstream Kuroshio, but new production below 50m was largely contributed by the vertical nutrient fluxes from below.
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固氮作用对黑潮上游区域生态系统影响的模拟研究
王艳君,董园,陈寅超,周卫文,李芊*
(中国科学院南海海洋研究所, 广东 广州 510301)
摘要:为了研究固氮作用对黑潮上游区域生态系统的影响,本文建立了一个包含固氮生物在内的 NPZD 生物
模型,初步模拟结果与观测结果相吻合。通过比较模型中有固氮和无固氮两种情况下,黑潮上游区域生态系
统各参量以及各级生产力的差异,揭示了该区域固氮生物的季节性分布特征,阐明了固氮作用对海洋生态系
统动力过程的重要影响。结果表明,固氮生物由于水文和化学因素的影响,主要出现在夏秋季节。固氮产生
的新氮源使黑潮上游区域硝酸盐、铵盐、浮游动植物和大小碎屑的量都有明显增加。浮游植物的平均生长速
率提高了大约 64%,初级生产力、再生生产力和新生产力分别增加了 30%左右。在夏季 50 米以浅水体,固氮
支持了 50%~80%的新生产力,是新生产力的主要贡献者;而在 50 米~200 米水体,固氮支持了 10%~50%的
新生产力,深层水的垂直混合带来的氮营养盐成为新生产力的主要贡献者。
关键词: NPZD 模型; 黑潮; 固氮; 生态系统; 新生产力
Modeling the impact of N2-fixation on the ecosystem dynamics in the
upstream Kuroshio
WANG Yan-jun, DONG Yuan, CHEN Yan-chao, ZHOU Wei-wen, LI Qian*
South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China
Abstract: A NPZD model with incorporation of a diazotroph function group was used to simulate the temporal
change of ecosystem in the upstream Kuroshio Current. The modeled nutrients, chlorophyll-a, and productivity
showed good agreements with the observations. The impacts of N2-fixation on nutrient concentrations, plankton
biomass, and detritus were assessed by comparisons of the modeled simulations with and without diazotrophs. Our
results suggested that the diazotrophs was most abundant in the summer and the fall when other phytoplankton were
nitrogen limited. The mean growth of phytoplankton could increased by 64% with the input of new nitrogen from
N2-fixation, leading to 30% increases of primary production, regenerated production and new production,
respectively. In the summer, N2-fixation supported 50%80% of the new production in the upper 50m, but only
10%50% in the depths of 50200m. This finding suggested that N2-fixation was an important source of new
nitrogen for phytoplankton production above 50m in the upstream Kuroshio, but new production below 50m was
largely contributed by the vertical nutrient fluxes from below.
Key words: NPZD model; Kuroshio; N2-fixation; ecosystem; new production
收稿日期: ;修订日期:
基金项目:中国科学院战略性先导科技专项(A 类)编号 XDA11020201-4
作者简介:王艳君(1982-)女,广东省广州市人,博士研究生,环境科学。E-mail:yanjun_wang82@126.com
通讯作者:李芊。E-mail:qianli@scsio.ac.cn
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1. 引言
海洋氮循环影响了海洋氮的生物可利用性,从而 (Vitousek and
Howarth, 1991; Zehr and Kudela, 2011)。生物固氮对海洋氮循环具有重要的作用(Capone et al,
1997)不但会造成生态系统从氮限制向磷限制的转变(Karl et al, 1992),还会对寡营养海域的
新生产力和碳循环产生重要的影响(Falkowski, 1997; Knapp, 2012)。已有研究表明,在北太平
洋夏威夷海洋时间序列观测站(ALOHA)生物固氮作用支持了高达50%的新生产力(Karl et al,
1997)而在北大西洋的百慕大时间序列站(BATS)固氮作用产生的氮通量则超过了由垂直混
合带到真光层的无机氮的通量,成为该海域新生产力重要来源(Capone et al, 2005)。位于西北
太平洋的黑潮水体,不但有较高的固氮生物量,而且影响了周围海域固氮生物的分布。研究
指出南海北部红海束毛藻的出现可能与黑潮水季节性入侵有关(王雨 等, 2012于黑潮水
的影响,东海也出现较高的固氮生物丰度,其固氮作用支持了约6%的初级生产力(杨清,
1998; Nakamura et al, 2005; 林峰 等, 2013近年来,虽然黑潮上游区生物固氮方面研究取得
了较大的进展,但是人们对于海洋固氮作用在较长时间尺度上是如何影响区域海水营养盐、
初级生产力、以及浮游生态系统等方面的认识还十分有限。研究这一课题将有助于我们更好
地理解固氮生物在黑潮上游区域氮循环中的重要地位。
海洋生态模拟作为研究海洋生态系统变化、评估固氮对海洋初级生产影响的有效工具已
被广泛研究(Fasham et al, 1990; Hood et al, 2001; Coles and Hood, 2007)Fennel(2002)建立
了一维的NPZ模型,很好地再现了北太平洋ALOHA站生物固氮的季节性和年际变化特点。
Hood(2004)利用三维的物理-生物(NPZD)耦合模型研究了大西洋束毛藻的分布和固氮
率,表明固氮生物和非固氮植物之间对光和营养盐的竞争在很大程度上控制着固氮作用的时
空变化。目前,关于海洋固氮模拟的研究大多集中于北大西洋和北太平洋的副热带环流区等
一些固氮明显的海域。而对固氮信号较强的黑潮上游区域的生态模拟研究则仍然鲜有报道。
本文以黑潮上游区的一个时间序列站点(122.5°E, 21.5°N,文中以K站表示)为研究对象,
立了包含氮循环和磷循环的NPZD模型并与一维物理模型ROMS嵌套,模拟在有固氮和无固
氮两种情况下,生态系统中各要素(包括营养盐、固氮生物、非固氮浮游植物、浮游动物、
大小碎屑等)的长期变化规律,分析固氮作用对该海域海洋生态系统的影响以及对初级生产
力的贡献。
2. 模型的介绍
2.1 NPZD模型的构建及各部分之间的相互关系
NPZD类型的模型是研究海洋生态系统动力学的一个基本工具。典型的NPZD模型中包含
营养盐(N、浮游植物(P、浮游动物(Z)和碎屑(D)四大模块。本文所建立的模型由8
个部分组成(图1包含三种营养盐(硝酸盐、磷酸盐和铵盐)两类浮游植物(固氮生物和
非固氮植物)、浮游动物、两类碎屑(大碎屑和小碎屑)。其中,小碎屑包含浮游动物和浮游
植物的尸体,以及动物摄食过程中未被同化的排泄物等。大碎屑则是指由小碎屑和浮游植物
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之间聚集而成的体积较大的碎屑。二者的矿化速率相同,而沉降速度存在显著的差异。物理
方面,模型中考虑了温度和光照对浮游植物生长的影响。
1包含有氮循环和磷循环的 NPZD 生物模型结构图。
Fig.1 Diagram of NPZD biological model compartments with nitrogen and phosphorus flows in the system.
影响非固氮浮游植物生长的营养盐很多,如铵盐、硝酸盐、磷酸盐、硅酸盐、铁、锌和
锰等。模型中只关注铵盐、硝酸盐和磷酸盐三种最重要的营养盐,设定非固氮浮游植物按照
一定的比率吸收氮和磷,并将铵盐作为首选的氮源,其次是硝酸盐,铵盐一方面能通过较低
的硝化速率转化为硝酸盐,另一方面也对浮游植物对硝酸盐的利用起着抑制的作用。这些营
养盐被浮游植物吸收利用并转化为有机物,浮游动物以浮游植物为食,其排泄物以及死后的
尸体,连同浮游植物的尸体都将被分解为小碎屑,小碎屑之间通过聚集变成体积较大的大碎
屑,大小碎屑由于沉降速率不同而分布于不同的水层,并最终被矿化为可溶性无机盐重新回
到水体,由此产生的再生营养盐绝大部分在真光层被浮游植物重新利用,转化为再生生产力
(Dugdale and Goering, 1967)。模型中的新生产力主要来自于深层水的垂直混合和固氮作用带
来的氮营养盐。
固氮生物和非固氮植物在模型中的区别主要体现在三个方面:不同的最大生长速率、不同
的氮源、以及不同的营养盐吸收率。固氮生物的生长速率不受海水氮营养盐浓度的限制,而
仅受磷酸盐浓度(Moutin et al, 2008; Wu, 2000)温度和光(Carpenter et al, 1993)的影响。固氮
生物对温度和光照要求比非固氮浮游植物更高,因此生长速度远低于非固氮浮游植物。非固
氮浮游植物利用水体中的可溶性无机氮作为氮源,固氮生物只能通过固定游离态的氮分子来
满足自身生长对氮源的需求。固氮生物固氮时,对营养盐按照45:1N/P比率进行吸收(Fennel
4
et al, 2002)155,而非固氮植物对营养盐的吸收则按照Redfield比值进行(Redfield, 1963),即N/P
16:1
2.2 模型所采用的主要公式及参数的选择
2.2.1 非固氮浮游植物和固氮生物的生长速率(
1
2)方程
 
 
pn NNNIT ,,, a
max
11
(1)
 
 
p
NIT
,
max
22
(2)
在方程(1)(2)中,
max
1
max
2
分别代表非固氮植物和固氮生物在营养盐饱和时的最大
生长速率,是温度和光照的函数。
(Nn,Na,Np)代表硝酸盐、铵盐和磷酸盐对非固氮浮游植物
生长的综合限制,即
(Nn,Na,Np) = min[
(Nn)+
(Na),
(Np)],固氮生物只受磷酸盐的限制,
(Np)表示。营养盐的限制方程
(Nn),
(Na) ,
(Np)
(Np)采用类似酶动力学的米氏方程
来表示(Fasham et al, 1990)597
)()(
)(
aa
a
nn
n
nKN
K
KN
N
N
(3)
)(
)(
aa
a
aKN
N
N
(4)
)(
)(
1pp
p
pKN
N
N
(5)
)(
)(
2pp
p
pKN
N
N
(6)
其中,Nn,Na,Np分别代表硝酸盐、铵盐和磷酸盐的浓度,Kn,Ka,Kp1分别代表非固氮
浮游植物利用硝酸盐、铵盐和磷酸盐的半饱和常数,Kp2代表固氮生物利用硝酸盐的半饱和常
数。
模型中还考虑了温度T和光照I对植物生长的限制,其限制方程 (Gruber et al, 2006)1489
下:
(7)
   
 
 
 
2
22
2
2
222
max
2,
IT
IT
IT
T
T
(8)
根据Eppley (1972),且固氮生物的生长速率相对于非固氮植物较小,得到如下方程:
 
T
TT066.159.0
1
(9)
 
T
TT066.112.0
2
(10)
式中,
1
2分别代表非固氮浮游植物和固氮生物P-I曲线的初始斜率,I代表光合有效辐
射,
1
2分别代表非固氮植物和固氮生物的叶绿素与碳的比值。此外,程序中所涉及的
5
它参量的状态方程在其它文章中有详细介绍(Gruber et al., 20061488,这里仅以附录的形式
呈现(见附录1
2.2.2 模型中主要参数的选择
选择合适的参数对模型的模拟效果至关重要,但是所采用的参数往往并不代表个体的生
理过程,而是反映群落水平的整体响应。模型中所选择的主要参数如下(见表1
1NPZD模型中采用的主要参数的值和单位
Tab.1 Values, units, symbols and definitions for parameters of the NPZD biological model
Parameter
Symbol
Value
Units
Half-sat. conc. for NO3 uptake (5)
Kn
0.8
mmol N m-3
Half-sat. conc. for NH4 uptake (3,5)
Ka
0.1
mmol N m−3
Half-sat. conc. for PO4 uptake by non-diazo
Kp1
0.05
mmol N m−3
Half-sat. conc. for PO4 uptake by diazotorph
Kp2
0.03
mmol N m−3
N:P ratio for non-diazotroph
r1
16
mmol N : mMol P
N:P ratio for diazotroph (4)
r2
45
mmol N : mMol P
Phytoplankton mortality to SDet rate (3)
P
0.07
day−1
Initial slope of P-I curve for non-diazotroph
1
7.0
mgC (mgChlaWm−2d)−1
Initial slope of P-I curve for diazotroph
2
4.0
mgC (mgChlaWm−2d)−1
Maximum chlorophyll to carbon ratio
max
0.041
mg Chla(mg C)−1
Zoo maximum growth rate (1,4)
gZ
1.0
day−1
Zoo assimilation efficiency (1,2,4)
Z
0.75
Zoo half-sat. constant for grazing (1,2)
KP
1.0
mmol N m-3
Zoo quadratic mortality to Detritus (2)
Z
0.1
day−1 (mmol N m-3)−1
Zoo specific excretion rate (1,3)
Z
0.1
day−1
Nitrification rate
knitr
0.1
day−1
Particle coagulation rate (2)
kc
0.005
day−1 (mmol N m-3)−1
Remineralization rate of DS(3)
r
S
k
0.1
day−1
Remineralization rate of DL(3)
r
L
k
0.1
day−1
Sinking velocity of phytoplankton (2)
P
0.5
m day−1
Sinking velocity of DS
S
0.5
m day−1
Sinking velocity of DL(2)
L
10
m day−1
Light attenuation coeff. for seawater (1,2)
kwater
0.04
m−1
Light attenuation coeff. for Chlorophyl (2)l
kchla
0.025
(m2mg Chla)-1
生物模型的参数设置所参考文献: (1) Fasham et al.(1990)602, (2) Gruber et al.(2006)1489 , (3) Troupin et al.(2010), (4) Fennel et
al.(2002)155, (5) Li et al. (2010).
2.2.3 NPZD模型与物理模型的嵌套
6
ROMS模型近年来已被广泛用于海洋模拟研究。该模式基于三维非线性斜压原始方程,
水平方向采用曲线正交坐标,垂直方向采用地形拟合的 s坐标。一ROMS模型包含着一个
物理-生物耦合体系和一个垂直混合层子模型(K-Profile Parameterization)。把所建立的含固氮
生物的NPZD模型与一维海洋物理模型(ROMS)嵌套(Troupin et al, 2010,可以模拟区
海洋生态系统的季节性和年际变化。
黑潮上游区域和南海相邻并通过吕宋口进行水体交换,是典型的氮限制寡营养海域。近
年来研究发现,该区域夏季具有着适合固氮生物生长的高温、低营养盐和水体层化较强等水
文特征。夏季较强的固氮作用支持了大量的新生产力,缓解了冬夏交替时由于硝酸盐被大量
消耗而导致初级生产力骤减的趋势,在支持远洋渔业资源方面发挥了巨大的作用。
本研究建立的一维NPZD-ROMS耦合模型,所采用的强迫场包括长波太阳辐射、短波太
阳辐射、海表空气温度、比湿、降水率、和海表10米高的风速等。利用观测的黑潮上游区域
温度、盐度、硝酸盐、磷酸盐和叶绿素a在冬季的垂向分布数据拟合曲线,对该站点的物理和
生物变量进行初始化设置。通过参数优化,模拟的海表温度和海表叶绿素a的长期变化与卫星
数据十分吻合,模拟的温度、盐度和密度的垂直分布也与观测结果一致。为了探讨固氮作用
如何影响黑潮上游区域生态系统的各个参量,本文在物理条件完全相同的情况下,进行了有
固氮生物存在和无固氮浮游植物存在两种模拟,并分析比较了两种模拟结果中生态系统各参
量的差异和生产力的变化。
3. 结果和讨论
3.1 模拟结果与观测结果的比较
营养盐、叶绿素和初级生产力分布的模拟结果和观测结果的比较如图 2所示。在考虑固
氮的情况下,模拟的硝酸盐、磷酸盐、叶绿素和生产力的分布与观测结果比较吻合(Chen et al,
2008)固氮作用的情况下,模拟结果与观测结果存在明显的差异。除表层的磷酸盐浓度略
高于观测结果外,其它参量的模拟结果总体上都比观测结果偏低,且冬季的偏差大于春季和
夏季。
有固氮作用时模拟的结果,表层硝酸盐和磷酸盐浓度均为 0.01 mmol·m-3 左右,与观测结
果相比;垂向分布上随深度的增加而增加,在 200 米处分别达到 2.8 0.18 mmol·m-3 左右。
在季节性分布上,混合层以下观测的硝酸盐和磷酸盐浓度有明显的季节性变化,即冬季>夏
季>春季。而模拟的硝酸盐和磷酸盐的季节性变化不如观测结果明显,呈现冬季浓度略高,
春季和夏季浓度相当(图 2a 2b)。模拟的表层叶绿素浓度(图 2c)为冬季(0.28 mg·m-3)
夏季 (0.1 mg·m-3) (0.06 mg·m-3)层叶 绿值呈 (0.341 mg·m-3)
(0.343 mg·m-3)高于冬季(0.2 mg·m-3)。与观测结果相比,次表层叶绿素极大值的浓度与观测结
果比较吻合。而表层叶绿素浓度略低于观测结果。
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2硝酸盐浓度、磷酸盐浓度、叶绿素 a浓度、初级生产力、硝酸盐支持的新生产力和固氮产生的新生产力
的季节性垂直分布的模拟结果与观测结果的比较。黑色粗线代表观测结果,蓝色细线为考虑固氮作用模拟的
结果,红色细线为无固氮作用模拟的结果。方形代表春季(2005 422 日至 52)圆形代表夏季2005
88日至 18 日),菱形代表冬季(2006 12 18 日至 30 )。观测结果来自 Chen et al. (2008)
Fig.2 Comparisons of the seasonal and vertical distributions of nitrate, phosphate, and Chlorophyll-a, primary
production, new production from nitrate, and new production from N2-fixation between the observed and the
modeled results for station K. Thick black lines are from observations, thin blue lines are from model with nitrogen
fixation and thin red lines are modeled results without N2 fixation; Squares are for spring (22 Apr-02 May 2005),
circles for summer (08-18 Aug 2005)and diamonds for winter(18-30 Dec 2006); Observed data were reproduced
from Chen et al. (2008).
模拟的初级生产力、硝酸盐支持的新生产力以及固氮支持的新生产力(2)在垂向分布上
与观测结果一致,均随深度的增加而减小。在季节性分布上,观测的初级生产力(2d)的季
节性变化(表层均为 13 mg C·m-3·d-1 左右)不明显。硝酸盐支持的新生产(2e)为冬季
高(3.8 mg C·m-3·d-1,春季和夏季(2 mg C·m-3·d-1 左右)相当。固氮支持的生产力(2f)
夏季最高(约为 0.85 mg C·m-3·d-1,春季和冬季都比较低。模拟的结果中,固氮支持的新生
产力的季节性分布与观测结果非常吻合,季最高值约为 0.62 mg C·m-3·d-1而初级生产力和
硝酸盐支持的新生产力在冬季的模拟结果与观测结果一致,春季和夏季略低于观测水平。比
较图 2e 和图 2f 可知,冬季和春季的新生产力主要由物理作用带到混合层的硝酸盐支持。夏
季的新生产力由物理作用和固氮作用带来的可溶性无机氮共同支持。观测结果中,夏季硝酸
盐支持的新生产力大于固氮支持的新生产力,是新生产力的主要来源。而模拟的结果显示,
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夏季由于水体层化加强且混合层较浅,硝酸盐支持的新生产力接近甚至略低于固氮支持的新
生产力。这也是模拟的初级生产力具有显著季节性差异的主要原因。
营养盐浓度对浮游植物和初级生产力的季节性变化具有显著的影响。由冬季季风引起的
强对流混合带到上层水体的硝酸盐促进了浮游植物,尤其是非固氮植物的生长(因为固氮生
物生长速度相对很低)随着营养盐的消耗和非固氮植物的减少,在夏季出现了可溶性无机氮
相对不足的情况,此时固氮生物通过固氮大量生长,一直延续至秋季。固氮为氮循环注入的
新氮和垂直混合带上来的可溶性无机氮又重新激发了冬季非固氮植物的生长(Hood et al,
2004)1
3.2 固氮作用对生态系统各个参量长期变化的影响
为了揭示黑潮上游区固氮作用对海洋生态系统中各参量的长期影响以及对海洋氮循环的
贡献,模型在物理条件和生物、化学参数完全相同的前提下,比较了有固氮和无固氮两种情
况的时间序列模拟结果。图 3和图 4分别描述了有固氮作用(右侧,fghij)和无固氮
作用(左侧,abcde)时生态系统各参量以及生产力的时空分布。
比较有固氮和无固氮模拟的结果表明:生态系统各参量在混合层以内有显著的差异(如
3和图 4所示)。在有固氮情况下,冬季表层的硝酸盐浓度大于 0.1 mmol·m-3,夏季的硝酸
盐浓度在 60 米处达到 0.10.2 mmol·m-3(图 3f。如果没有固氮,冬季表层的硝酸盐浓度只
0.05 mmol·m-3 左右,夏季的硝酸盐浓度在 60 米处为 0.040.08 mmol·m-3
(图 3a无论是
否有固氮作用,冬季混合层以内的磷酸盐浓度变化不大(图 3b 3g而夏季表层的磷酸盐浓
度在有固氮时为 0.01 mmol·m-3(图 3g,约是无固氮时磷酸盐浓度(0.02 mmol·m-3)的 1/2
(图 3b。冬季表层铵盐浓度在有固氮情况下为 0.04 mmol·m-3(图 3h,是无固氮时铵盐浓
度的两倍(图 3c。次表层铵盐的极大值在有固氮情况下为 0.25 mmol·m-3 左右,略大于无固
氮情况下的铵盐浓度(0.2 mmol·m-3
浮游植物的生物量和营养盐浓度紧密地联系在一起。随着新氮的输入,非固氮植物的表
层叶绿素 a(图 3d 3i)在冬季和夏季分别由 0.2 0.03 mg·m-3
(图 3d升高0.25 0.05
mg·m-3
(图 3i次表层叶绿素极大值相比无固氮时也增加了 0.03mg·m-3 左右。固氮生物的生
长受温度、光照和可溶性无机营养盐等条件的影响。硝酸盐浓度较低的海表,固氮生物的生
长主要受温度的控制和磷限制。在温度低于 26C的冬春季节,固氮生物很少,生物量均低
0.005 mmol N·m-3。而在温度介于 26C~30C之间的夏秋两季,固氮生物的生物量和温度
呈现一定的正相关关系(P2= 0.003T- 0.074 ,r2= 0.2828, p<0.01)。在 2005 9月、2006 9
月和 2012 9月,固氮生物的生物量最高达到约 0.048~0.05 mmol N·m-3,叶绿素 a最高达
0.03 mg·m-3。然而,随着深度的增加,光照强度逐渐减小,硝酸盐浓度逐渐增加,固氮生物
与其它浮游植物相比处于劣势,在 50 米左右由 0.05 mmol N·m-3 降低为 0.01 mmol N·m-3,在
硝酸盐跃层(75±5m)降为 0.001 mmol N·m-3 左右(2012 9月)。适合固氮生物大量繁殖
的夏秋两季(图 3j,比硝酸盐和铵盐浓度大幅增长的冬季提前了 45个月左右。这是因为
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固氮生物从生长、被摄食、沉降、分解、再矿化,到氮的输出并被浮游植物利用,这一新氮
源的循环需要一个过程(Coles et al, 2004)在这一循环过程中,浮游植物的迅速生长刺激了浮
游动物以及大小碎屑的增加。浮游动物的大量繁殖和和大小碎屑的浓度高值主要集中在 1
份至 7月份,与无固氮的情况相比浓度明显增加。其中,浮游动物的生物量由无固氮时
0.03~0.21 mmol N·m-3 增加为有固氮时的 0.2~0.33 mmol N·m-3小碎屑由 0.18~0.24 mmol N·m-3
增加为 0.25~0.34 mmol N·m-3;大碎屑由 0.003~0.005 mmol N·m-3 增加为 0.005~0.01 mmol
N·m-3碎屑经过矿化作用转化为更多的无机态的铵盐和磷酸盐,重新被植物吸收利用,最终
带来较高的生产力。
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3在考虑固氮(右列, fghij)和不考虑固氮(左列 abcde两种情况下,模型模拟的硝酸
盐浓度、磷酸盐浓度、铵盐浓度、非固氮植物和固氮生物叶绿素 a浓度从 2007 年至 2012 年的时空分布。
Fig.3 Time-series distributions of nitrate, phosphate, ammonium, and chlorophyll-a modeled with (right, fghi
j) and without (left, abcde) N2-fixation in station K from 2007 to 2012
200 米以内水体,有固氮作用时的硝酸盐和铵盐的积分,分别是不考虑固氮时二者积
分的 1.21 INO3_固氮 =1.21* INO3_无固氮+4.06r2=0.993n=342721.47 INH4_固氮 =1.47* INH4_
无固氮+0.576r2=0.922n=34272。由于固氮生物固氮时是按照 45:1 的比率吸收氮和磷的,所
以磷酸盐的积分变化不太明显。叶绿素 a的积分相比无固氮的情况增加了约 14%IChla_固氮
=1.14* IChla_无固氮+1.62r2=0.981n=34272
总之,相比于无固氮的情况,有固氮作用时的硝酸盐、铵盐、浮游植物、浮游动物以及
大小碎屑的量都有明显增加。由于冬季垂直混合加强,冬季的增加幅度高于夏季(Liu et al,
2002)。而磷酸盐的浓度由于固氮生物的吸收利用在夏季表层有明显减少。
3.3 固氮作用对初级生产力、再生生产力和新生产力的影响
由图 4可知,无论是否考虑固氮作用的影响,硝酸盐支持的新生产力(NO3_Nprd、再
生生产力(Rprd)以及初级生产力都具有相似的分布特征。即季节性分布上,冬季高于其它
季节。垂向分布上,冬季表层最高并随深度的增加而减小。夏季呈现双峰分布,有明显的次
表层极值现象。固氮产生的新生产力Nfix_Nprd(图 4i)与固氮生物的季节性变化相一致,
主要出现在夏秋两季,最高值可达 0.79 mg C·m-3·d-12010 8月)
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4在考虑固氮(右列, fghi)和不考虑固氮(左列 abcd)两种情况下,模型模拟的初级生产
(mg C·m-3·d-1)、再生生产力(mg C·m-3·d-1)、硝酸盐产生的新生产力(mg C·m-3·d-1)和固氮产生的新生产力(mg
C·m-3·d-1)2007 年至 2012 年的时空分布。
Fig.4 Time-series distributions of primary production (mg C·m-3·d-1), regenerate production (mg C·m-3·d-1), new
production from nitrate (mg C·m-3·d-1), and new production from N2-fixation (mg C·m-3·d-1) modeled with (right, f
ghi) and without (left, abcd) N2-fixation in station K from 2007 to 2012.
相比于无固氮的情况,有固氮时各级生产力全年都有提高,冬季增幅更为显著。冬季表
层的 Pprd Rprd NO3_Nprd(图 4f4g 4h最高值分别达到了 15.611.0 4.3 mg C·m-3·d-1
2011 1月),分别是不考虑固氮作用时最高值(10.56.61 3.59 mg C·m-3·d-11.5
倍、1.66 倍和 1.2 倍(图 4a4b 4c。在 0200 米水柱中,初级生产力的积分(IPprd
酸盐支持的新生产力的积分(INO3_Nprd)和再生生产力的积分(IRprd)均为不考虑固氮作用时
1.3 倍左右(r20.85n=34272。也就是说,如果该区域没有固氮作用为氮循环输入的这
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部分新氮源,IPprdINO3_Nprd IRprd 将分别降低近 30%。这与 Coles(2004)等模拟的北大西洋固
氮作用对新生产力的增加幅度(30%)相当,但低于亚热带北太平洋固氮作用贡献的新生产
力份额(50%
在夏季 050 米水柱中,固氮产生的新生产力积分占总新生产力积分的 50%80%,而
50200 米之间水柱,约占 10%50%说明夏50 米以浅水体中,固氮作用是支持新生
产力的主体,50 米以深水体,扩散作用是支持新生产力的主体。这是因为夏季,水体层化加
强,次表层硝酸盐的通量较低,0~50 米之间的物理扩散作用不足以支持新生产力,此时固氮
生物大量生长,成为新生产力的重要贡献者。随着深度的增加,固氮生物丰度减小,固氮作
用随之降低。在 50 米以深水体,物理扩散作用输送的硝酸盐支持了大部分的新生产力。
INO3_Nprd IRprd 均 与 IPprd 具有很好的正相关关系,在不考虑固氮作用时
INO3_Nprd=0.230*IPprd-0.706r2=0.701n=34272,即 f ratio (INO3_Nprd/IPprd)约为 23%加入固
氮作用后,INO3_Nprd=0.249*IPprd-11.0r2=0.835n=34272f ratio 增加了近 2%。说明固氮作
用对新生产力的影响程度略大于其对再生生产力的影响。
另外,浮游植物的生物量Phy)和初级生产力Pprd)之间也存在着正相关关系。不考
虑固氮时,Pprd=0.166* Phy-0.004 (r2=0.427n=34272)。加入固氮时,Pprd=0.272* Phy-0.007
(r2=0.458n=34272)也就是说,固氮作用使浮游植物的平均生长速率由 0.166d-1 增加为 0.272
d-1,增幅为 64%。 倘若不考虑固氮,而只将浮游植物的最大生长速率设置为原来的两倍
模拟的初级生产力虽有提高却仍低于现有的有固氮时的生产力水平。这一结果反映出,在氮
受限的黑潮上游区域,营养盐中氮的浓度是限制海洋生态系统中生产力水平的关键因素。单
纯提高浮游植物的生长率虽能小幅度地提高初级生产力,却远不抵固氮作用对整个生态系统
的影响深远。
综上可见,在黑潮上游区域,固氮生物自身生长所支持的新生产力,对于支持海洋初级
生产力具有至关重要的作用。它所固定的有机氮通过再循环被非固氮植物所利用,使浮游植
物的生长速率和各级生产力水平都明显提高。
4. 结论和展望
本研究所建立的一维物理-生物耦合模型很好地模拟出了黑潮上游区域生态系统中硝酸
盐、磷酸盐、叶绿素和生产力的长期变化规律,再现了由可溶性无机氮浓度变化引起的固氮
生物和非固氮植物交替出现的季节性分布,这种分布规律与观测结果非常吻合。通过比较有
无固氮作用两种情况下,黑潮上游区域生态系统中各变量的变化规律可知,由固氮作用输入
的新氮源,增加了海水无机氮盐的浓度,缓解了浮游植物群落的氮限制,使平均生长速率提
高了约 64%同时,它刺激了浮游植物、浮游动物和大小碎屑的级联正响应,使初级生产力、
再生生产力和新生产力分别提高约 30%。总之,在氮受限的黑潮上游区,固氮作用对海洋生
态系统具有十分重要的意义。
依据早期对主要固氮生物束毛藻的研究和认识,模型中只考虑了生活在真光层中的固氮
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生物,并且认为固氮速度在海水温暖和氮营养盐缺乏的表层水域中最大。近年来通过现场观
测、实验室培养以及分子生物学手段研究还发现了许多新型的固氮生物。例如,在硝酸盐浓
度达到 20µM 的热带南太平洋海表(Fernandez et al, 2011)和水温低于 19℃的北太平洋真光层
水域(Needoba et al, 2007),都观测到有较强的固氮信号。如果将这些研究发现在模型中加
考虑,那么模型中的部分参数将需要进一步优化。
海洋生态系统各要素之间的关系和物质能量的流动是一个错综复杂的过程。在今后的研
究中,我们将在模型中增加更多的变量,浮游植物将按照粒径细化为硅藻、甲藻和蓝细菌,
在营养盐方面增加硅酸盐和铁的限制,同时在浮游动物中增加大型的浮游动物和鱼类,以期
更加全面地描述多营养级的食物网,更加准确地模拟海洋生态系统的变化
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15
附录1NPZD模型中采用的主要方程
Appendix 1 Equations of the NPZD biological model
Variables
Equations
Photosynthetica
lly available
radiation
 
iiichlawaterii zChlaChlakkII ]21[5.0exp
1
Non-diazotroph
)(),,(),( 111
21
1
1
max
1
1
s
c
P
P
ZPan DPPkP
PP
P
ZgPNNNIT
t
P
Diazotroph
2
21
2
2
max
2
2)(),( P
PP
P
ZgPNIT
t
P
P
P
ZP
Zooplankton
2
21
21 ZZ
PP
PP
Zg
t
Z
ZZ
P
ZZ
Small detritus
z
D
DkDPDkZPP
PP
PP
Zg
t
DS
Ss
r
SSS
c
ZP
P
ZZ
S
)()()1( 1
2
21
21
21
Large detritus
z
D
DkDPk
t
DL
LL
r
LS
c
L
2
1)(
Nitrate
a
nitr
an
n
pan
nNIkP
NN
N
NNNIT
t
N
)(
),(
)(
),,(),( 1
max
1
Ammonium
L
r
Ls
r
SZa
nitr
an
a
pan
aDkDkZNIkP
NN
N
NNNIT
dt
dN
)(
),(
)(
),,(),( 1
max
1
Phosphate
1112
2
max
2
1
1
max
1)(),(),,(),( r
D
k
r
D
k
r
Z
r
P
NIT
r
P
NNNIT
t
NL
r
L
S
r
SZppan
p
Chl-a:C for
non-diazotroph
 
 
1
2
11
2
1
max
1
max
1
1
)(
),,()(
),,(),(
IT
NNNT
NNNIT
tT
pan
T
pan
Chl-a:C for
diazotroph
 
 
2
2
22
2
2
max
2
max
2
2
)(
)()(
)(),(
IT
NT
NIT
tT
p
T
p
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